Periodic patterns and technique to control misalignment between two layers

ABSTRACT

A method and system to measure misalignment error between two overlying or interlaced periodic structures are proposed. The overlying or interlaced periodic structures are illuminated by incident radiation, and the diffracted radiation of the incident radiation by the overlying or interlaced periodic structures are detected to provide an output signal. The misalignment between the overlying or interlaced periodic structures may then be determined from the output signal.

BACKGROUND OF THE INVENTION

[0001] The invention relates in general to metrology systems formeasuring periodic structures such as overlay targets, and, inparticular, to a metrology system employing diffracted light fordetecting misalignment of such structures.

[0002] Overlay error measurement requires specially designed marks to bestrategically placed at various locations, normally in the scribe linearea between dies, on the wafers for each process. The alignment of thetwo overlay targets from two consecutive processes is measured for anumber of locations on the wafer, and the overlay error map across thewafer is analyzed to provide feedback for the alignment control oflithography steppers.

[0003] A key process control parameter in the manufacturing ofintegrated circuits is the measurement of overlay target alignmentbetween successive layers on a semiconductor wafer. If the two overlaytargets are misaligned relative to each other, then the electronicdevices fabricated will malfunction, and the semiconductor wafer willneed to be reworked or discarded.

[0004] Measurement of overlay misregistration between layers is beingperformed today with optical microscopy in different variations:brightfield, darkfield, confocal, and interference microscopy, asdescribed in Levinson, “Lithography Process Control,” chapter 5, SPIEPress Vol. TT28, 1999. Overlay targets may comprise fine structures ontop of the wafer or etched into the surface of the wafer. For example,one overlay target may be formed by etching into the wafer, whileanother adjacent overlay target may be a resist layer at a higherelevation over the wafer. The target being used for this purpose iscalled box-in-box where the outer box, usually 10 to 30 μm, representsthe position of the bottom layer, while the inner box is smaller andrepresents the location of the upper layer. An optical microscopic imageis grabbed for this target and analyzed with image processingtechniques. The relative location of the two boxes represents what iscalled the overlay misregistration, or the overlay. The accuracy of theoptical microscope is limited by the accuracy of the line profiles inthe target, by aberrations in the illumination and imaging optics and bythe image sampling in the camera. Such methods are complex and theyrequire full imaging optics. Vibration isolation is also required.

[0005] These techniques suffer from a number of drawbacks. First, thegrabbed target image is highly sensitive to the optical quality of thesystem, which is never ideal. The optical quality of the system mayproduce errors in the calculation of the overlay misregistration.Second, optical imaging has a fundamental limit on resolution, whichaffects the accuracy of the measurement. Third, an optical microscope isa relatively bulky system. It is difficult to integrate an opticalmicroscope into another system, such as the end of the track of alithographic stepper system. It is desirable to develop an improvedsystem to overcome these drawbacks.

SUMMARY OF THE INVENTION

[0006] A target for determining misalignment between two layers of adevice has two periodic structures of lines and spaces on the twodifferent layers of a device. The two periodic structures overlie or areinterlaced with each other. The layers or periodic structures may be atthe same or different heights. In one embodiment, either the firstperiodic structure or the second periodic structure has at least twosets of interlaced grating lines having different periods, line widthsor duty cycles. The invention also relates to a method of makingoverlying or interlaced targets.

[0007] An advantage of the target is the use of the same diffractionsystem and the same target to measure critical dimension and overlaymisregistration. Another advantage of the measurement of misregistrationof the target is that it is free from optical asymmetries usuallyassociated with imaging.

[0008] The invention also relates to a method of detecting misalignmentbetween two layers of a device. The overlying or interlaced periodicstructures are illuminated by incident radiation. The diffractedradiation from the overlying or interlaced periodic structures is usedto provide an output signal. In one embodiment, a signal is derived fromthe output signal. The misalignment between the structures is determinedfrom the output signal or the derived signal. In one embodiment, theoutput signal or the derived signal is compared with a reference signal.A database that correlates the misalignment with data related todiffracted radiation can be constructed.

[0009] An advantage of this method is the use of only one incidentradiation beam. Another advantage of this method is the high sensitivityof zero-order and first-order diffracted light to the overlaymisregistration between the layers. In particular, properties whichexhibited high sensitivity are intensity, phase and polarizationproperties of zero-order diffraction; differential intensity between thepositive and negative first-order diffraction; differential phasebetween the positive and negative first-order diffraction; anddifferential polarization between the positive and negative first-orderdiffraction. These properties also yielded linear graphs when plottedagainst the overlay misalignment. This method can be used to determinemisalignment on the order of nanometers.

[0010] In one embodiment, a neutral polarization angle, defined as anincident polarization angle where the differential intensity is equal tozero for all overlay misregistrations, is determined. The slope ofdifferential intensity as a function of incident polarization angle ishighly linear when plotted against the overlay misregistration. Thislinear behavior reduces the number of parameters that need to bedetermined and decreases the polarization scanning needed. Thus, themethod of detecting misalignment is faster when using the slopemeasurement technique.

[0011] The invention also relates to an apparatus for detectingmisalignment of overlying or interlaced periodic structures. Theapparatus comprises a source, at least one analyzer, at least onedetector, and a signal processor to determine misalignment of overlyingor interlaced periodic structures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIGS. 1a-1 h are cross-sectional views illustrating basic processsteps in semiconductor processing.

[0013]FIG. 2a is a cross-sectional view of two overlying periodicstructures.

[0014]FIGS. 2b and 2 c are top views of the two overlying periodicstructures of FIG. 2a.

[0015]FIG. 3 is a top view of two overlying periodic structuresillustrating an embodiment of the invention.

[0016]FIGS. 4a and 4 b are cross-sectional views of overlying orinterlaced periodic structures illustrating other embodiments of theinvention.

[0017]FIG. 5a and 5 b are cross-sectional views of two interlacedperiodic structures illustrating interlaced gratings in an embodiment ofthe invention.

[0018]FIG. 6 is a cross-sectional view of two interlaced periodicstructures illustrating interlaced gratings in another embodiment of theinvention.

[0019]FIGS. 7a and 7 b are schematic views illustrating negative andpositive overlay shift, respectively.

[0020]FIG. 8 is a schematic view illustrating the diffraction of lightfrom a grating structure.

[0021]FIG. 9a is a schematic block diagram of an optical system thatmeasures zero-order diffraction from overlying or interlaced periodicstructures.

[0022]FIG. 9b is a schematic block diagram of an integrated system ofthe optical system of FIG. 9a and a deposition instrument.

[0023]FIGS. 10a and 11 a are schematic block diagrams of an opticalsystem that measures first-order diffraction from a normal incident beamon overlying or interlaced periodic structures.

[0024]FIGS. 10b and 11 b are schematic block diagrams of integratedsystems of the optical systems of FIGS. 10a and 11 a, respectively, anda deposition instrument.

[0025]FIGS. 12a and 12 b are graphical plots of derived signals fromzero-order diffraction of incident radiation on overlying structures.

[0026] FIGS. 13-14 and 16-17 are graphical plots of derived signals fromfirst-order diffraction of incident radiation on overlying structures.

[0027]FIG. 15 is a graphical plot illustrating the mean square error.

[0028] FIGS. 18-19 and 21-22 are graphical plots of derived signals fromzero-order diffraction of incident radiation on interlaced gratings.

[0029]FIGS. 20 and 23 are graphical plots illustrating the mean squareerror.

[0030]FIG. 24 is a graphical plot illustrating the determination ofmisalignment from a slope near a neutral polarization angle.

[0031] For simplicity of description, identical components are labeledby the same numerals in this application.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0032]FIG. 2a is a cross-sectional view of a target 11 comprising twoperiodic structures 13, 15 on two layers 31, 33 of a device 17. Thesecond periodic structure 15 is overlying or interlaced with the firstperiodic structure 13. The layers and the periodic structures may be atthe same or different heights. The device 17 can be any device of whichthe alignment between two layers, particularly layers having smallfeatures on structures, needs to be determined. These devices aretypically semiconductor devices; thin films for magnetic heads for datastorage devices such as tape recorders; and flat panel displays.

[0033] As shown in FIGS. 1a-1 h, a device 17 is generally formed in abasic series of steps for each layer. First, as shown in FIG. 1a, alayer 2 is formed on a semiconductor substrate 1. The layer 2 may beformed by oxidization, diffusion, implantation, evaporation, ordeposition. Second, as shown in FIG. 1b, resist 3 is deposited on thelayer 2. Third, as shown in FIG. 1c, the resist 3 is selectively exposedto a form of radiation 5. This selective exposure is accomplished withan exposure tool and mask 4, or data tape in electron or ion beamlithography (not shown). Fourth, as shown in FIG. 1d, the resist 3 isdeveloped. The resist 3 protects the regions 6 of the layer 2 that itcovers. Fifth, as shown in FIG. 1e, the exposed regions 7 of the layer 2are etched away. Sixth, as shown in FIG. 1f, the resist 3 is removed.Alternatively, in another embodiment, another material 8 can bedeposited in the spaces 7, as shown in FIG. 1e, of the etched layer 2,as shown in FIG. 1g, and the resist 3 is removed after the deposition,as shown in FIG. 1h. This basic series of steps is repeated for eachlayer until the desired device is formed.

[0034] A first layer 31 and a second layer 33 can be any layer in thedevice. Unpatterned semiconductor, metal or dielectric layers may bedeposited or grown on top of, underneath, or between the first layer 31and the second layer 33.

[0035] The pattern for the first periodic structure 13 is in the samemask as the pattern for a first layer 31 of the device, and the patternfor the second periodic structure 15 is in the same mask as the patternfor a second layer 33 of the device. In one embodiment, the firstperiodic structure 13 or the second periodic structure 15 is the etchedspaces 7 of the first layer 31 or the second layer 33, respectively, asshown in FIG. 1f. In another embodiment, the first periodic structure 13or the second periodic structure 15 is the lines 2 of the first layer 31or the second layer 33, respectively, as shown in FIG. 1f. In anotherembodiment, the first periodic structure 13 or the second periodicstructure 15 is another material 8 deposited in the spaces 7 of thefirst layer 31 or the second layer 33, respectively, as shown in FIG.1h. In yet another embodiment, the second layer 33 is resist, and thesecond periodic structure 15 is resist 3 gratings, as shown in FIG. 1d.

[0036] The first periodic structure 13 has the same alignment as thefirst layer 31, since the same mask was used for the pattern for thefirst periodic structure 13 and for the pattern for the first layer 31.Similarly, the second periodic structure 15 has the same alignment asthe second layer 33. Thus, any overlay misregistration error in thealignment between the first layer 31 and the second layer 33 will bereflected in the alignment between the first periodic structure 13 andthe second periodic structure 15.

[0037]FIGS. 2b and 2 c are top views of target 11. In one embodiment, asillustrated in FIG. 2a, the first periodic structure 13 has a firstselected width CD1, and the second periodic structure 15 has a secondselected width CD2. The second selected width CD2 is less than the firstselected width CD1. The pitch, also called the period or the unit cell,of a periodic structure is the distance after which the pattern isrepeated. The distance between the left edge of the first periodicstructure 13 and the left edge of the second periodic structure 15 isd₁, and the distance between the right edge of the first periodicstructure 13 and the right edge of the second periodic structure 15 isd₂. In a preferred embodiment, when layers 31, 33 are properly alignedrelative to each other, the second periodic structure 15 is centeredover the first periodic structure 13. In other words, when the secondperiodic structure 15 is perfectly centered over the first periodicstructure 13, the misregistration is zero, and d₁=d₂. In thisembodiment, the misregistration is indicated by d₂−d₁. To obtainmisregistration in both the X and Y directions of the XY coordinatesystem, another target 12 comprising two periodic structures 14, 16similar to target 11 is placed substantially perpendicular to target 11,as shown in FIG. 2c.

[0038] The target 11 is particularly desirable for use inphotolithography, where the first layer 31 is exposed to radiation forpatterning purposes of a semiconductor wafer and the second layer 33 isresist. In one embodiment, the first layer 31 is etched silicon, and thesecond layer 33 is resist.

[0039]FIGS. 4a and 4 b show alternative embodiments. In one embodiment,FIG. 4a illustrates a first periodic structure 13 of oxide having atrapezoidal shape on a first layer 31 of silicon substrate and a secondperiodic structure 15 of resist with a second layer 33 of resist. Thefirst layer 31 of silicon is etched, and shallow trench isolation(“STI”) oxide is deposited in the spaces of the etched silicon. Thelines of STI oxide form the first periodic structure 13. An oxide layer34 and a uniform polysilicon layer 35 are deposited between the firstlayer 31 of silicon and the second layer 33 of resist. The configurationin FIG. 4a shows a line on space configuration, where the secondperiodic structure 15 is placed aligned with the spaces between thefirst periodic structure 13. The invention also encompasses embodimentssuch as the line on line configuration, where the lines in the secondperiodic structure 15 are placed on top of and aligned with the lines inthe first periodic structure 13, as shown by the dotted lines in FIG.4a.

[0040] In another embodiment, FIG. 4b illustrates a first periodicstructure 13 of tungsten etched in a first layer 31 of oxide and asecond periodic structure 15 of resist with a second layer 33 of resist.The first layer 31 and the second layer 33 are separated by an aluminumblanket 37.

[0041] The invention relates to a method of making a target 11. A firstperiodic structure 13 is placed over a first layer 31 of a device 17. Asecond periodic structure 15 is placed over a second layer 33 of thedevice 17. The second periodic structure 15 is overlying or interlacedwith the first periodic structure 13.

[0042] In one embodiment, another target 12 is placed substantiallyperpendicular to target 11, as shown in FIG. 2c. A third periodicstructure 14 is placed over the first layer 31, and a fourth periodicstructure 14 is placed over the second layer 33. The third periodicstructure 14 is substantially perpendicular to the first periodicstructure 13, and the fourth periodic structure 16 is substantiallyperpendicular to the second periodic structure 15.

[0043] An advantage of the target 11 is that the measurement ofmisregistration of the target is free from optical asymmetries usuallyassociated with imaging. Another advantage of this measurement is thatit does not require scanning over the target as it is done with othertechniques, such as in Bareket, U.S. Pat. No. 6,023,338. Anotheradvantage of the target 11 is the elimination of a separate diffractionsystem and a different target to measure the critical dimension (“CD”)of a periodic structure. The critical dimension, or a selected width ofa periodic structure, is one of many target parameters needed tocalculate misregistration. Using the same diffraction system and thesame target to measure both the overlay misregistration and the CD ismore efficient. The sensitivity associated with the CD and that with themisregistration is distinguished by using an embodiment of a target asshown in FIG. 3. The second periodic structure 15 extends further to anarea, the CD region 21, where the first periodic structure 13 does notextend. The first selected width CD1 is measured before placing thesecond periodic structure 15 on the device 17. After forming the target,the second selected width CD2 alone can be measured in the CD region 21.In a separate measurement, the misregistration is determined in anoverlay region 19 where both the first 13 and second 15 periodicstructures lie.

[0044]FIGS. 5a and 5 b are cross-sectional views of an embodiment of atarget having interlaced gratings. The first periodic structure 13 orthe second periodic structure 15 has at least two interlaced gratinglines having different periods, line widths or duty cycles. The firstperiodic structure 13 is patterned with the same mask as that for thefirst layer 31, and the second periodic structure 15 is patterned withthe same mask as that for the second layer 33. Thus, the first periodicstructure 13 has the same alignment as the first layer 31, and thesecond periodic structure 15 has the same alignment as the second layer33. Any misregistration between the first layer 31 and the second layer33 is reflected in the misregistration between the first periodicstructure 13 and the second periodic structure 15.

[0045] In the embodiment shown in FIGS. 5a and 5 b, the first periodicstructure 13 has two interlaced grating lines 51, 53. The firstinterlaced grating lines 51 have a line-width L₁, and the secondinterlaced grating lines 53 have a line-width L₂. The second periodicstructure 15, as shown in FIG. 5b, has a line-width L₃ and is centeredbetween the first interlaced grating lines 51 and the second interlacedgrating lines 53. The distance between the right edge of the firstinterlaced grating 51 and the adjacent left edge of the secondinterlaced grating 53 is represented by b, and the distance between theright edge of the second periodic structure 15 and the adjacent leftedge of the second interlaced grating 53 is represented by c. Themisregistration between the first layer 31 and the second layer 33 isequal to the misregistration ε between the first periodic structure 13and the second periodic structure 15. The misregistration ε is:$\begin{matrix}{ɛ = {\frac{b}{2} - \frac{L_{3}}{2} - c}} & (1)\end{matrix}$

[0046] Where c=0, the resulting periodic structure has the mostasymmetric unit cell composed of a line with width of L₂+L₃ and a linewith width L₁. Where c=b−L₃, the resulting periodic structure has themost symmetric unit cell composed of a line with width L₁+L₃ and a linewith width L₂. For example, if the two layers are made of the samematerial and L₁=L₃=L₂/2, then the lines are identical where c=0, whileone line is twice as wide as the other line where c=b−L₃.

[0047]FIG. 6 shows an alternative embodiment of a target havinginterlaced gratings. The first periodic structure 13 is etched silicon,and the second periodic target 15 is resist. The first layer 31 ofsilicon substrate and the second layer 33 of resist are separated by anoxide layer 39.

[0048] The invention also relates to a method of making a target 11. Afirst periodic structure 13 is placed over a first layer 31 of a device17. A second periodic structure 15 is placed over a second layer 33 ofthe device 17. The second periodic structure 15 is overlying orinterlaced with the first periodic structure 13. Either the firstperiodic structure 13 or the second periodic structure 15 has at leasttwo interlaced grating lines having different periods, line widths orduty cycles.

[0049] An advantage of interlaced gratings is the ability to determinethe sign of the shift of the misregistration from the symmetry of theinterlaced gratings. FIGS. 7a and 7 b are schematic drawingsillustrating negative and positive overlay shift, respectively, in the Xdirection of the XY coordinate system. Center line 61 is the center of agrating 63. When the grating 63 is aligned perfectly, the center line 61is aligned with the Y axis of the XY coordinate system. As shown in FIG.7a, a negative overlay shift is indicated by the center line 61 being inthe negative X direction. As shown in FIG. 7b, a positive overlay shiftis indicated by the center line 61 being in the positive X direction.The negative overlay shift is indicated by a negative number for themisregistration, and the positive overlay shift is indicated by apositive number for the misregistration. The misregistration can bedetermined using the method discussed below. In the case of theinterlaced gratings, a negative overlay shift results in a moresymmetrical unit cell, as where c=b−L₃, discussed above. A positiveoverlay shift results in a more asymmetrical unit cell, as where c=0,discussed above.

[0050] The invention relates to a method to determine misalignment usingdiffracted light. FIG. 8 is a schematic view showing the diffraction oflight from a grating structure 71. In one embodiment, incident radiation73 having an oblique angle of incidence θ illuminates the gratingstructure 71. The grating structure 71 diffracts radiation 75, 77, 79.Zero-order diffraction 75 is at the same oblique angle θ to thesubstrate as the incident radiation 73. Negative first-order diffraction77 and positive first-order diffraction 79 are also diffracted by thegrating structure 71.

[0051] Optical systems for determining misalignment of overlying orinterlaced periodic structures are illustrated in FIGS. 9a, 10 a, and 11a. FIG. 9a shows an optical system 100 using incident radiation beam 81with an oblique angle of incidence and detecting zero-order diffractedradiation 83. A source 102 provides polarized incident radiation beam 81to illuminate periodic structures on a wafer 91. The incident radiationbeam may be substantially monochromatic or polychromatic. The source 102comprises a light source 101 and optionally acollimating/focusing/polarizing optical module 103. The structuresdiffract zero-order diffracted radiation 83. Acollimating/focusing/analyzing optical module 105 collects thezero-order diffracted radiation 83, and a light detection unit 107detects the zero-order diffracted radiation 83 collected by the analyzerin module 105 to provide an output signal 85. A signal processor 109determines any misalignment between the structures from the outputsignal 85. The output signal 85 is used directly to determinemisalignment from the intensity of the zero-order diffracted radiation83. In a preferred embodiment, the misalignment is determined bycomparing the intensity with a reference signal, such as a referencesignal from a calibration wafer or a database, compiled as explainedbelow. In one embodiment, the signal processor 109 calculates a derivedsignal from the output signal 85 and determines misalignment from thederived signal. The derived signal can include polarization or phaseinformation. In this embodiment, the misalignment is determined bycomparing the derived signal with a reference signal.

[0052] In one embodiment, optical system 100 provides ellipsometricparameter values, which are used to derive polarization and phaseinformation. In this embodiment, the source 102 includes a light source101 and a polarizer in module 103. Additionally, a device 104 causesrelative rotational motion between the polarizer in module 103 and theanalyzer in module 105. Device 104 is well known in the art and is notdescribed for this reason. The polarization of the reflected light ismeasured by the analyzer in module 105, and the signal processor 109calculates the ellipsometric parameter values, tan(Ψ) and cos(Δ), fromthe polarization of the reflected light. The signal processor 109 usesthe ellipsometric parameter values to derive polarization and phaseinformation. The phase is Δ. The polarization angle α is related totan(Ψ) through the following equation: $\begin{matrix}{{\tan \quad \alpha} = \frac{1}{\tan \quad \Psi}} & (2)\end{matrix}$

[0053] The signal processor 109 determines misalignment from thepolarization or phase information, as discussed above.

[0054] The imaging and focusing of the optical system 100 in oneembodiment is verified using the vision and pattern recognition system115. The light source 101 provides a beam for imaging and focusing 87.The beam for imaging and focusing 87 is reflected by beam splitter 113and focused by lens 111 to the wafer 91. The beam 87 then is reflectedback through the lens 111 and beam splitter 113 to the vision andpattern recognition system 115. The vision and pattern recognitionsystem 115 then sends a recognition signal 88 for keeping the wafer infocus for measurement to the signal processor 109.

[0055]FIG. 10a illustrates an optical system 110 using normal incidentradiation beam 82 and detecting first-order diffracted radiation 93, 95.A source 202 provides polarized incident radiation beam 82 to illuminateperiodic structures on a wafer 91. In this embodiment, the source 202comprises a light source 101, a polarizer 117 and lens 111. Thestructures diffract positive first-order diffracted radiation 95 andnegative first-order diffracted radiation 93. Analyzers 121, 119 collectpositive first-order diffracted radiation 95 and negative first-orderdiffracted radiation 93, respectively. Light detection units 125, 123detect the positive first-order diffracted radiation 95 and the negativefirst-order diffracted radiation 93, respectively, collected byanalyzers 121, 119, respectively, to provide output signals 85. A signalprocessor 109 determines any misalignment between the structures fromthe output signals 85, preferably by comparing the output signals 85 toa reference signal. In one embodiment, the signal processor 109calculates a derived signal from the output signals 85. The derivedsignal is a differential signal between the positive first-orderdiffracted radiation 95 and the negative first-order diffractedradiation 93. The differential signal can indicate a differentialintensity, a differential polarization angle, or a differential phase.

[0056] Optical system 110 determines differential intensity,differential polarization angles, or differential phase. To determinedifferential phase, optical system 110 in one embodiment uses anellipsometric arrangement comprising a light source 101, a polarizer117, an analyzer 119 or 121, a light detector 123 or 125, and a device104 that causes relative rotational motion between the polarizer 117 andthe analyzer 119 or 121. Device 104 is well known in the art and is notdescribed for this reason. This arrangement provides ellipsometricparameters for positive first-order diffracted radiation 95 andellipsometric parameters for negative first-order diffracted radiation93, which are used to derive phase for positive first-order diffractedradiation 95 and phase for negative first-order diffracted radiation 93,respectively. As discussed above, one of the ellipsometric parameters iscos(Δ), and the phase is Δ. Differential phase is calculated bysubtracting the phase for the negative first-order diffracted radiation93 from the phase for the positive first-order diffracted radiation 95.

[0057] To determine differential polarization angles, in one embodiment,the polarizer 117 is fixed for the incident radiation beam 82, and theanalyzers 121, 119 are rotated, or vice versa. The polarization anglefor the negative first-order diffracted radiation 93 is determined fromthe change in intensity as either the polarizer 117 or analyzer 119rotates. The polarization angle for the positive first-order diffractedradiation 95 is determined from the change in intensity as either thepolarizer 117 or analyzer 121 rotates. A differential polarization angleis calculated by subtracting the polarization angle for the negativefirst-order diffracted radiation 93 from the polarization angle for thepositive first-order diffracted radiation 95.

[0058] To determine differential intensity, in one embodiment, theanalyzers 119, 121 are positioned without relative rotation at thepolarization angle of the first-order diffracted radiation 93, 95.Preferably, at the polarization angle where the intensity of thediffracted radiation is a maximum, the intensity of the positivefirst-order diffracted radiation 95 and the intensity of the negativefirst-order diffracted intensity 93 is detected by the detectors 125,123. Differential intensity is calculated by subtracting the intensityfor the negative first-order diffracted radiation 93 from the intensityfor the positive first-order diffracted radiation 95.

[0059] In another embodiment, the differential intensity is measured asa function of the incident polarization angle. In this embodiment, thepolarizer 117 is rotated, and the analyzers 119, 121 are fixed. As thepolarizer 117 rotates, the incident polarization angle changes. Theintensity of the positive first-order diffracted radiation 95 and theintensity of the negative first-order diffracted radiation 93 isdetermined for different incident polarization angles. Differentialintensity is calculated by subtracting the intensity for the negativefirst-order diffracted radiation 93 from the intensity for the positivefirst-order diffracted radiation 95.

[0060] The imaging and focusing of the optical system 110 in oneembodiment is verified using the vision and pattern recognition system115. After incident radiation beam 82 illuminates the wafer 91, a lightbeam for imaging and focusing 87 is reflected through the lens 111,polarizer 117, and beam splitter 113 to the vision and patternrecognition system 115. The vision and pattern recognition system 115then sends a recognition signal 88 for keeping the wafer in focus formeasurement to the signal processor 109.

[0061]FIG. 11a illustrates an optical system 120 where first-orderdiffracted radiation beams 93, 95 are allowed to interfere. The lightsource 101, device 104, polarizer 117, lens 111, and analyzers 119, 121operate the same way in optical system 120 as they do in optical system110. Device 104 is well known in the art and is not described for thisreason. Once the negative first-order diffracted radiation 93 andpositive first-order diffracted radiation 95 are passed through theanalyzers 119, 112, respectively, a first device causes the positivefirst-order diffracted radiation 95 and the negative first-orderdiffracted radiation 93 to interfere. In this embodiment, the firstdevice comprises a multi-aperture shutter 131 and a flat beam splitter135. The multi-aperture shutter 131 allows both the negative first-orderdiffracted radiation 93 and the positive first-order diffracted beam 95to pass through it. The flat beam splitter 135 combines the negativefirst-order diffracted radiation 93 and the positive first-orderdiffracted radiation 95. In this embodiment, the mirrors 127, 133 changethe direction of the positive first-order diffracted radiation 95. Alight detection unit 107 detects the interference 89 of the twodiffracted radiation signals to provide output signals 85. A signalprocessor 109 determines any misalignment between the structures fromthe output signals 85, preferably by comparing the output signals 85 toa reference signal. The output signals 85 contain information related tophase difference.

[0062] In one embodiment, phase shift interferometry is used todetermine misalignment. The phase modulator 129 shifts the phase ofpositive first-order diffracted radiation 95. This phase shift of thepositive first-order diffracted radiation 95 allows the signal processor109 to use a simple algorithm to calculate the phase difference betweenthe phase for the positive first-order diffracted radiation 95 and thephase for the negative first-order diffracted radiation 93.

[0063] Differential intensity and differential polarization angle canalso be determined using optical system 120. The multi-aperture shutter131 operates in three modes. The first mode allows both the positivefirst-order diffracted radiation 95 and the negative first-orderdiffracted radiation 93 to pass through. In this mode, differentialphase is determined, as discussed above. The second mode allows only thepositive first-order diffracted radiation 95 to pass through. In thismode, the intensity and polarization angle for the positive first-orderdiffracted radiation 95 can be determined, as discussed above. The thirdmode allows only the negative first-order diffracted radiation 93 topass through. In this mode, the intensity and polarization angle for thenegative first-order diffracted radiation 93 can be determined, asdiscussed above.

[0064] To determine differential intensity, the multi-aperture shutter131 is operated in the second mode to determine intensity for positivefirst-order diffracted radiation 95 and then in the third mode todetermine intensity for negative first-order diffracted radiation 93, orvice versa. The differential intensity is then calculated by subtractingthe intensity of the negative first-order diffracted radiation 93 fromthe intensity of the positive first-order diffracted radiation 95. Thesignal processor 109 determines misalignment from the differentialintensity.

[0065] In one embodiment, the differential intensity is measured atdifferent incident polarization angles. The measurements result in alarge set of data points, which, when compared to a reference signal,provide a high accuracy in the determined value of the misregistration.

[0066] To determine differential polarization angle, the multi-apertureshutter 131 is operated in the second mode to determine polarizationangle for positive first-order diffracted radiation 95 and then in thethird mode to determine polarization angle for negative first-orderdiffracted radiation 93, or vice versa. The differential polarizationangle is then calculated by subtracting the polarization angle of thenegative first-order diffracted radiation 93 from the polarization angleof the positive first-order diffracted radiation 95. The signalprocessor 109 determines misalignment from the differential polarizationangle.

[0067] The imaging and focusing of the optical system 120 is verifiedusing the vision and pattern recognition system 115 in the same way asthe imaging and focusing of the optical system 110 is in FIG. 10. In oneembodiment, the beam splitter 113 splits off radiation 89 to referencelight detection unit 137, which detects fluctuations of the light source101. The reference light detection unit 137 communicates information 86concerning intensity fluctuation of source 101 to the signal processingand computing unit 109. The signal processor 109 normalizes the outputsignal 85 using fluctuation information 86.

[0068] Optical systems 100, 110, 120 can be integrated with a depositioninstrument 200 to provide an integrated tool, as shown in FIGS. 9b, 10 band 11 b. The deposition instrument 200 provides the overlying orinterlaced periodic structures on wafer 91 in step 301. Optical systems100, 110, 120 obtains misalignment information from the wafer 91 in step302. The signal processor 109 of optical systems 100, 110, 120 providesthe misalignment to the deposition tool 200 in step 303. The depositiontool uses the misalignment information to correct for any misalignmentbefore providing another layer or periodic structure on wafer 91 in step301.

[0069] Optical systems 100, 110, 120 are used to determine themisalignment of overlying or interlaced periodic structures. The sourceproviding polarized incident radiation beam illuminates the firstperiodic structure 13 and the second periodic structure 15. Diffractedradiation from the illuminated portions of the overlying or interlacedperiodic structures are detected to provide an output signal 85. Themisalignment between the structures is determined from the output signal85. In a preferred embodiment, the misalignment is determined bycomparing the output signal 85 with a reference signal, such as areference signal from a calibration wafer or a database, compiled asexplained below.

[0070] The invention relates to a method for providing a database todetermine misalignment of overlying or interlaced periodic structures.The misalignment of overlying or interlaced periodic structures andstructure parameters, such as thickness, refractive index, extinctioncoefficient, or critical dimension, are provided to calculate datarelated to radiation diffracted by the structures in response to a beamof radiation. The data can include intensity, polarization angle, orphase information. Calculations can be performed using known equationsor by a software package, such as Lambda SW, available from Lambda,University of Arizona, Tuscon, Ariz., or Gsolver SW, available fromGrating Solver Development Company, P.O. Box 353, Allen, Tex. 75013.Lambda SW uses eigenfunctions approach, described in P. Sheng, R. S.Stepleman, and P. N. Sandra, Exact Eigenfunctions for Square WaveGratings: Applications to Diffraction and Surface Plasmon Calculations,Phys.Rev. B, 2907-2916 (1982), or the modal approach, described in L.Li, A Modal Analysis of Lamellar Diffraction Gratings in ConicalMountings, J. Mod. Opt. 40, 553-573 (1993). Gsolver SW uses rigorouscoupled wave analysis, described in M. G. Moharam and T. K. Gaylord,Rigorous Coupled-Wave Analysis of Planar-Grating Diffraction, J. Opt.Soc. Am. 73, 1105-1112 (1983). The data is used to construct a databasecorrelating the misalignment and the data. The overlay misregistrationof a target can then be determined by comparing the output signal 85with the database.

[0071] FIGS. 12-24 were generated through computer simulations usingeither the Lambda SW or the Gsolver SW. FIGS. 12a and 12 b are graphicalplots illustrating the ellipsometric parameters obtained using anoverlying target of FIG. 2a with the optical system of FIG. 9a. Thecalculations were performed using the Lambda SW. The overlying targetused in the measurement comprises first periodic structure 13 and thesecond periodic structure 15 made of resist gratings having 1 μm depthon a silicon substrate. The depth of the first periodic structure 13 andthe second periodic structure 15 is 0.5 μm, and the pitch is 0.8 μ. Thefirst selected width CD1 for the first periodic structure 13 is 0.4 μm,and the second selected width CD2 for the second periodic structure 15is 0.2 μm. The incident beam in this embodiment was TE polarized. Thesetarget parameters and the overlay misregistration were inputted into theLambda SW to obtain ellipsometeric parameter values. The ellipsometricparameter values were obtained for zero-order diffracted radiation usingan incident radiation beam 81 at an angle of 25° to the wafer surface.The ellipsometric parameters, Tan[Ψ] and Cos[Δ], were plotted as afunction of the wavelengths in the spectral range 230 to 400 nanometers.The ellipsometric parameters are defined as: $\begin{matrix}{{\tan \quad \Psi} = \frac{r_{p}}{r_{s}}} & (3)\end{matrix}$

[0072] where r_(p) and r_(s) are the amplitude reflection coefficientsfor the p(TM) and s(TE) polarizations, and

Δ=φ_(p)−φ_(s)  (4)

[0073] where φ_(p) and φ_(s) are the phases for the p(TM) and s(TE)polarizations. Results were obtained for different values of overlaymisregistration d₂−d₁ varying from −15 nanometers to 15 nanometers insteps of 5 nanometers. The variations for tan[Ψ] and cos[Δ] showsensitivity to the misregistration in the nanometer scale. To get moreaccurate results, first-order diffracted radiation is detected usingnormal incident radiation, as in FIGS. 13-14.

[0074]FIGS. 13 and 14 are graphical plots illustrating the differentialintensity obtained using overlying targets of FIG. 2a and an opticalsystem detecting first-order diffracted radiation using normal incidentradiation. The calculations were performed using Gsolver SW. The firstperiodic layer 13 is etched silicon, while the second periodic layer 15is resist. The overlay misregistration and target parameters wereinputted into Gsolver SW to obtain the differential intensity in FIGS.13 and 14. FIG. 13 shows the normalized differential intensity betweenthe positive and negative first-order diffracted radiation as a functionof overlay misregistrations. The differential intensity is defined as:$\begin{matrix}{{DS} = {\frac{R_{+ 1} - R_{- 1}}{R_{+ 1} + R_{- 1}}\%}} & (5)\end{matrix}$

[0075] where R₊₁ is the intensity of the positive first-order diffractedradiation and R⁻¹ is the intensity of the negative first-orderdiffracted radiation. The different curves in FIG. 13 correspond to thedifferent incident polarization angles (0°, 50°, 60°, 74°, 80°, and 90°)of the incident linearly polarized light relative to the plane ofincidence. The polarization angle α is defined as: $\begin{matrix}{\alpha = {\arctan ( \frac{E_{s}}{E_{p}} )}} & (6)\end{matrix}$

[0076] where E_(s) is the field component perpendicular to the plane ofincidence, which for normal incidence is the Y component in the XYcoordinate system, and E_(p) is the field component parallel to theplane of incidence, which for normal incidence is the X component.Polarization scans from incident polarization angles of 0° to 90° wereperformed to generate the graphical plots in FIGS. 13 and 14. FIG. 14shows the differential intensity as a function of incident polarizationangle at different overlay misregistration (−50 nm, −35 nm, −15 nm, 0nm, 15 nm, 35 nm, and 50 nm). FIG. 14 shows that there is a neutralpolarization angle, defined as an incident polarization angle where thedifferential intensity is equal to zero for all overlay misregistration.FIGS. 13 and 14 illustrate the high sensitivity of differentialintensity to the overlay misregistration and the linear behavior ofdifferential intensity with the overlay misregistration. They also showthat the differential intensity is zero at zero overlay misregistrationfor any polarization angle. Similar graphical plots were obtained atdifferent wavelengths. FIG. 15 shows the mean square error (“MSE”)variation with the overlay misregistration. The MSE exhibits linearityand sensitivity of approximately 0.6 per one nanometer overlaymisregistration.

[0077]FIGS. 16 and 17 are graphical plots, using the same target withdifferent structure parameters and the same optical system as the onesin FIGS. 13 and 14. However, the calculations were performed using theLambda SW, instead of the Gsolver SW. The kinks or the deviations fromthe montonicity of the curves at certain points in FIGS. 16 and 17 arebelieved to be due to numerical instabilities frequently known to occurin the use of the Lambda SW. The overlay misregistration and the targetparameters were inputted into Lambda SW to obtain differentialpolarization angle and differential phase in FIGS. 16 and 17,respectively. FIG. 16 shows the variation of the difference between thepolarization angles of the positive and negative first-order diffractedradiation as a function of overlay misregistration for differentincident polarization angles (0°, 5°, 15°, 30°, 45°, 60°, and 90°). FIG.17 shows the variation of the difference between the phase angles of thepositive and negative first-order diffracted radiation. The phase anglehere represents the phase difference between the p and s polarizedcomponents of the diffracted light.

[0078]FIGS. 16 and 17 also illustrate the high sensitivity ofdifferential polarization angle and differential phase, respectively, tothe overlay misregistration and the linear behavior of differentialpolarization angle and differential phase, respectively, when plottedagainst the overlay misregistration. They also show that thedifferential polarization angle and differential phase is zero at zerooverlay misregistration for any polarization angle. However, FIG. 17shows that the phase difference does not depend on incidentpolarization. In one embodiment, the difference between the polarizationangles, as shown in FIG. 16, is easily measured with an analyzer at theoutput, while the phase difference, as shown in FIG. 17, is measuredwith interferometry. In another embodiment, the differentialpolarization angle and the differential phase is derived fromellipsometric parameters.

[0079] Similar results were obtained using the overlying targets inFIGS. 4a and 4 b. However, for the particular target in FIG. 4a, therewas no neutral polarization angle in the line on line configuration,where the second periodic structure 15 is centered on the first periodicstructure 13. The line on space configuration, where the second periodicstructure 15 is centered on the spaces between the first periodicstructure 13, did exhibit a neutral polarization angle. These resultsshow that the neutral polarization angle apparently has a complicateddependence on the structure parameters.

[0080] FIGS. 18-19 and 21-22 are graphical plots illustrating theintensity of the zero-order diffracted radiation 83, as shown in FIG.9a, for interlaced gratings, as shown in FIG. 6. Table 1 summarizes theparameters used in the calculations by the Gsolver SW. TABLE 1 Structureparameters used in the simulations Parameter Data76 Data0 h1  850 nm 850 nm h2  850 nm  850 nm h3  600 nm  600 nm Pitch (P) 1000 nm 2000 nmCD1  150 nm  200 nm CD2  300 nm  600 nm CD3  150 nm  200 nm Incidenceangle (θ) 76° 0 Azimuth angle (φ)  0 0 Wavelength (λ)  670 nm  500 nm

[0081] The incidence angle is 76° in the Data76 configuration, and theincidence angle is 0° (normal) in the Data0 configuration.

[0082] FIGS. 18-20 were derived using the Data76 configuration. FIG. 18shows the intensity of the zero-order diffracted radiation versus theoverlay misregistration at different polarization angles (0° to 90° insteps of 15°). Within a range of 140 nm, the changes are monotonic withthe overlay misregistration. The point where all the curves cross is atan overlay misregistration value of 50 nm, rather than zero. At anoverlay misregistration value of 50 nm, the structure is effectivelymost symmetric. In contrast, in an overlying target as in FIG. 2a, thestructure is most symmetric at zero overlay misregistration. FIG. 19shows the dependence of the intensity of the zero-order diffractedradiation on the incident polarization angle at different overlaymisregistrations (−50 nm, −15 nm, 0 nm, 20 nm, 40 nm, 60 nm, 80 nm, 100nm, and 130 nm). Unlike with the differential intensity of thefirst-order diffracted radiation, there is not a neutral polarizationangle where the differential intensity is zero for different overlaymisregistration. However, there is a quasi-neutral polarization anglewhere most of the curves for different misregistration cross. FIG. 20shows the MSE variation as a function of overlay misregistration. FIGS.18 and 19 show the high sensitivity of the intensity of zero-orderdiffracted radiation to the overlay sign for a configuration usingincident radiation having an oblique angle of incidence on interlacedgratings. They also show the linear behavior of the intensity whenplotted against the overlay misregistration.

[0083] FIGS. 21-23 were derived using the Data0 configuration. FIG. 21shows the intensity of the zero-order diffracted radiation versus theoverlay misregistration at different polarization angles (0°, 40°, 65°,and 90°). FIG. 22 shows the dependence of the intensity of thezero-order diffracted radiation on the incident polarization angle atdifferent overlay misregistrations (−140 nm, −100 nm, −50 nm, 0 nm, 50nm, and 100 nm). FIG. 23 shows the MSE variation as a function ofoverlay misregistration. FIGS. 21 and 22 show the high sensitivity ofthe intensity of zero-order diffracted radiation to the overlay sign fora configuration using normal incident radiation on interlaced gratings.They also show the linear behavior of the intensity when plotted againstthe overlay misregistration.

[0084]FIG. 24 is a graphical plot generated by the Gsolver SWillustrating the determination of misalignment from the neutralpolarization angle. As shown in FIG. 14, the differential intensityequals zero independent of the overlay misregistration at the neutralpolarization angle. However, the slope of the differential intensityvaries with overlay misregistration. FIG. 24 shows the slope near theneutral polarization angle as a function of overlay misregistration.FIG. 24 shows linear behavior of the slope versus the overlaymisregistration with a slope of 0.038% per 1 nm overlay misregistration.An advantage of the slope measurement technique is the reduction of thenumber of parameters that need to be determined. Another advantage isthe decreased polarization scanning needed. In FIG. 14, a polarizationscan using incident polarization angles from 0° to 90° is performed. Incontrast, using the slope measurement technique in one embodiment, thederived signal is compared with the reference signal for polarizationangles within about five degrees of the neutral polarization angle.Thus, the method of detecting misalignment is faster when using theslope measurement technique. Another embodiment of the invention is theuse of the slope measurement technique for the quasi-neutralpolarization angle.

[0085] Misalignment of overlying or interlaced periodic structures canbe determined using the database in a preferred embodiment. The sourceproviding polarized incident radiation illuminates the first periodicstructure 13 and the second periodic structure 15. Diffracted radiationfrom the illuminated portions of the overlying or interlaced periodicstructures are detected to provide an output signal 85. The outputsignal 85 is compared with the database to determine the misalignmentbetween the overlying or interlaced periodic structures.

[0086] In another embodiment, misalignment of overlying or interlacedperiodic structures is determined using the slope measurement technique.A neutral polarization angle or quasi-neutral polarization angle isprovided. The derived signal is compared with the reference signal nearthe neutral polarization angle or the quasi-neutral polarization angleto determine misalignment of the overlying or interlaced periodicstructures.

[0087] While the invention has been described above by reference tovarious embodiments, it will be understood that changes andmodifications may be made without departing from the scope of theinvention, which is to be defined only by the appended claims and theirequivalent. All references referred to herein are incorporated byreference.

What is claimed is:
 1. A target for measuring the relative positionsbetween two layers of a device, said target comprising: a first periodicstructure over a first layer of the device; and a second periodicstructure over a second layer of the device, said second periodicstructure overlying or interlaced with said first periodic structure. 2.The target of claim 1, wherein the first periodic structure has a firstselected width, and the second periodic structure has a second selectedwidth, the second selected width being less than the first selectedwidth.
 3. The target of claim 1, wherein said second periodic structureextends further to an area where said first periodic structure does notextend.
 4. The target of claim 1, wherein the first layer is etchedsilicon, and the second layer is resist.
 5. The target of claim 1,wherein said first periodic structure has a trapezoidal shape, the firstlayer is silicon dioxide, and the second layer is resist, the firstlayer and the second layer being separated by an uniform polysiliconlayer.
 6. The target of claim 1, wherein said first periodic structureis tungsten and has a concave-trapezoidal shaped top, the first layer isoxide, and the second layer is resist, the first layer and the secondlayer being separated by an aluminum blanket.
 7. The target of claim 1,further comprising unpatterned semiconductor, metal, or dielectriclayers deposited or grown on top of, underneath, or between the firstand the second layers.
 8. The target of claim 1, wherein a layer that isthe topmost layer is resist.
 9. The target of claim 1, wherein the firstperiodic structure has been exposed to radiation for patterning purposesof a semiconductor wafer.
 10. The target of claim 1, further comprising:a third periodic structure that is substantially perpendicular to saidfirst periodic structure, said third periodic structure over the firstlayer; and a fourth periodic structure that is substantiallyperpendicular to said second periodic structure, said fourth periodicstructure over the second layer and overlying or interlaced with saidthird periodic structure.
 11. The target of claim 1, wherein said firstperiodic structure has at least two interlaced grating lines havingdifferent periods, line widths or duty cycles.
 12. The target of claim1, wherein said second periodic structure has at least two interlacedgrating lines having different periods, line widths or duty cycles. 13.A method for making a target, comprising: placing a first periodicstructure over a first layer of a device; and placing a second periodicstructure over a second layer of a device, wherein said second periodicstructure is overlying or interlaced with said first periodic structure.14. The method of claim 13, wherein said placing a second periodicstructure includes placing said second periodic structure on an area towhere said first periodic structure does not extend.
 15. The method ofclaim 13, further comprising exposing the first periodic structure toradiation for patterning purposes of a semiconductor wafer.
 16. Themethod of claim 13, further comprising: placing a third periodicstructure over the first layer, wherein said third periodic structure issubstantially perpendicular to said first periodic structure; andplacing a fourth periodic structure over the second layer, wherein saidfourth periodic structure is substantially perpendicular to said secondperiodic structure.
 17. The method of claim 13, wherein said placing afirst periodic structure includes placing at least two interlacedgrating lines having different periods, line widths or duty cycles. 18.The method of claim 13, wherein said placing a second periodic structureincludes placing at least two interlaced grating lines having differentperiods, line widths or duty cycles.
 19. A method for providing adatabase to determine misalignment of overlying or interlaced periodicstructures, comprising: providing information related to thickness,refractive index, extinction coefficient, or critical dimension, andmisalignment of periodic structures that overly or interlace oneanother; deriving from said information data related to radiationdiffracted by the structures in response to a beam of radiation; andconstructing a database correlating the misalignment and the data. 20.The method of claim 19, further comprising calculating a differentialintensity, a differential phase, or a differential polarization anglefrom the data.
 21. A method for detecting misalignment of overlying orinterlaced periodic structures, comprising: illuminating the overlyingor interlaced periodic structures with incident radiation; detectingdiffracted radiation from the illuminated portions of the overlying orinterlaced periodic structures to provide an output signal; anddetermining a misalignment between the structures from the outputsignal.
 22. The method of claim 21, wherein said determining includescomparing the output signal with a reference signal.
 23. The method ofclaim 22, wherein the reference signal comprises a database.
 24. Themethod of claim 21, wherein the output signal contains informationrelated to ellipsometric parameters.
 25. The method of claim 21, whereinoverlying or interlaced periodic structures has at least two interlacedgrating lines having different periods, line widths or duty cycles; theincident radiation is incident on the structures at an oblique angle;and the diffracted radiation comprises zero-order diffraction.
 26. Themethod of claim 21, wherein overlying or interlaced periodic structureshas at least two interlaced grating lines having different periods, linewidths or duty cycles; the incident radiation is incident on thestructures at a normal angle; and the diffracted radiation compriseszero-order diffraction.
 27. The method of claim 21, wherein the incidentradiation is substantially normal, and the diffracted radiationcomprises positive first-order diffraction and negative first-orderdiffraction.
 28. The method of claim 21, further comprising calculatinga derived signal from the output signal.
 29. The method of claim 28,wherein the derived signal contains information related to intensity,phase, or polarization angle.
 30. The method of claim 28, wherein thederived signal contains information related to differential intensity,differential phase, or differential polarization angle.
 31. The methodof claim 28, further comprising providing a neutral polarization angleor quasi-neutral polarization angle; and wherein said determining amisalignment includes determining a misalignment by comparing thederived signal with the reference signal near the neutral polarizationangle or the quasi-neutral polarization angle.
 32. The method of claim31, wherein the derived signal is compared with the reference signal forpolarization angles within about five degrees of the neutralpolarization angle or the quasi-neutral polarization angle.
 33. Anapparatus for detecting misalignment of overlying or interlaced periodicstructures, comprising: a source providing polarized incident radiationbeam to illuminate the overlying or interlaced periodic structures; atleast one analyzer collecting diffracted radiation from the structures;at least one detector detecting diffracted radiation collected by theanalyzer to provide output signals; and a signal processor determiningany misalignment between the structures from the output signals.
 34. Theapparatus of claim 33, wherein the source provides incident radiationbeam having an oblique angle of incidence to illuminate the overlying orinterlaced periodic structures, and the detector detects zero-orderdiffraction.
 35. The apparatus of claim 33, wherein the source providesa normal incident radiation beam to illuminate the overlying orinterlaced periodic structures, and the detector detects zero-orderdiffraction.
 36. The apparatus of claim 33, wherein the source includesa polarizer and a device causing relative rotational motion between thepolarizer and the analyzer.
 37. The apparatus of claim 33, wherein saidat least one analyzer comprises a first analyzer collecting positivefirst-order diffracted radiation and a second analyzer collectingnegative first-order diffracted radiation; and said at least onedetector comprises a first detector detecting positive first-orderdiffracted radiation, and a second detector detecting negativefirst-order diffracted radiation.
 38. The apparatus of claim 37, whereinthe signal processor calculates a derived signal from the outputsignals.
 39. The apparatus of claim 38, wherein the derived signalcontains information related to a differential intensity, a differentialphase, or a differential polarization angle.
 40. The apparatus of claim38, wherein the source includes a polarizer and a device causingrelative rotational motion between the polarizer and the analyzers. 41.The apparatus of claim 40, wherein the derived signal containsinformation related to a differential polarization angle or a phasedifference derived from ellipsometric parameters.
 42. An apparatus fordetecting misalignment of overlying or interlaced periodic structures,comprising: a source providing polarized incident radiation beam toilluminate the overlying or interlaced periodic structures; twoanalyzers collecting first-order diffracted radiation from thestructures, the first-order diffracted radiation comprising a positivefirst-order diffraction and a negative first-order diffraction; a firstdevice interfering the positive first-order diffraction and the negativefirst order diffraction from the analyzers to provide a combineddiffracted radiation signal; a detector detecting the combineddiffracted radiation signal to provide output signals; and a signalprocessor determining any misalignment between the structures from theoutput signals.
 43. The apparatus of claim 42, wherein the output signalcontains information related to phase difference between the positivefirst-order diffraction and the negative first-order diffraction.
 44. Anapparatus for making overlying or interlaced periodic structures anddetecting misalignment between the overlying or interlaced periodicstructures, comprising: a deposition instrument to provide the overlyingor interlaced periodic structures; a source providing polarized incidentradiation beam to illuminate the overlying or interlaced periodicstructures; at least one analyzer collecting diffracted radiation fromthe structures; at least one detector detecting diffracted radiationcollected by the analyzer to provide output signals; and a signalprocessor determining any misalignment between the structures from theoutput signals and providing the misalignment to the depositioninstrument.
 45. The apparatus of claim 44, wherein the source providesan incident radiation beam having an oblique angle of incidence toilluminate the overlying or interlaced periodic structures, and thedetector detects zero-order diffraction.
 46. The apparatus of claim 44,wherein the source provides a normal incident radiation beam toilluminate the overlying or interlaced periodic structures, and thedetector detects zero-order diffraction.
 47. The apparatus of claim 44,wherein the source includes a polarizer and a device causing relativerotational motion between the polarizer and the analyzer.
 48. Theapparatus of claim 44, wherein said at least one analyzer comprises afirst analyzer collecting positive first-order diffracted radiation anda second analyzer collecting negative first-order diffracted radiation;and said at least one detector comprises a first detector detectingpositive first-order diffracted radiation, and a second detectordetecting negative first-order diffracted radiation.
 49. The apparatusof claim 48, wherein the signal processor calculates a derived signalfrom the output signals.
 50. The apparatus of claim 49, wherein thederived signal contains information related to a differential intensity,a differential phase, or a differential polarization angle.
 51. Theapparatus of claim 49, wherein the source includes a polarizer and adevice causing relative rotational motion between the polarizer and theanalyzers.
 52. The apparatus of claim 51, wherein the derived signalcontains information related to a differential polarization angle or aphase difference derived from ellipsometric parameters.
 53. An apparatusfor making overlying or interlaced periodic structures and detectingmisalignment between the overlying or interlaced periodic structures,comprising: a deposition instrument to provide the overlying orinterlaced periodic structures; a source providing polarized incidentradiation beam to illuminate the overlying or interlaced periodicstructures; two analyzers collecting first-order diffracted radiationfrom the structures, the first-order diffracted radiation comprising apositive first-order diffraction and a negative first-order diffraction;a first device interfering the positive first-order diffraction and thenegative first order diffraction from the analyzers to provide acombined diffracted radiation signal; a detector detecting the combineddiffracted radiation signal to provide output signals; and a signalprocessor determining any misalignment between the structures from theoutput signals and providing the misalignment to the depositioninstrument.
 54. The apparatus of claim 53, wherein the output signalcontains information related to phase difference between the positivefirst-order diffraction and the negative first-order diffraction.